The invention relates to a process for preparing emulsions of organopolysiloxanes by polymerization of hydroxyl-terminated organopolysiloxanes and trialkyl-silyl-terminated organopolysiloxanes.
Preparing emulsions of organopolysiloxanes by emulsion polymerization is a long-established method. This applies particularly to the emulsion polymerization of cyclics. U.S. Pat. No. 2,891,920, for instance, describes the preparation of organopolysiloxane emulsions of low oil viscosity from octamethylcyclotetrasiloxane in the presence of strong alkalis or strong mineral acids.
DE 1495512 describes the simultaneous use of sulphonic acids and salts thereof as an emulsifier and a catalyst for emulsion polymerization of cyclic dimethylsiloxanes at different temperatures. However, the cyclic content of the resulting product is generally above 10%.
Different catalysts/emulsifiers/stabilizers for the purpose of obtaining more stable emulsions or emulsions of desired oil viscosity are described in many patents, for example DE 1495512. Particularly stable emulsions are stated therein to be obtainable by forming a pre-emulsion in particular.
Preparing emulsions of organopolysiloxanes in the presence of trimethylsilyl-terminated monomers is described in JP 2002-37888 and also JP 2000-95661. Similarly, U.S. Pat. No. 2,891,920 already describes this method for the eventuality that trimethylsilyl-terminated dimethylsiloxanes are to be obtained; cyclic dimethyl-siloxanes are emulsion polymerized therein together with hexamethyldisiloxane in a molar ratio of 99:1.
Preparing emulsions of organopolysiloxanes from hydroxyl-terminated organopolysiloxanes and cyclic organopolysiloxanes in an aqueous medium in the presence of a short-chain trialkylsilyl-terminated diorganopolysiloxane mandated to have a viscosity of not more than 50 mm2/s is described in JP 2006-117868.
This review shows that preparing emulsions of organopolysiloxanes by emulsion polymerization is very complex and—in relation to setting a suitable chain length and hence a suitable oil viscosity coupled with optimal emulsion properties such as shear and storage stability—difficult. This is mainly because the mechanism of an emulsion polymerization involves chain propagation and chain breakage taking place concurrently.